1. Field of the Invention
The present invention relates to an organic-inorganic composite material formed from an organic polymer and a metal alkoxide, and a laminate, an optical waveguide and a light transmission structure including the composite material.
2. Related Art
Inorganic materials such as metal and ceramics are excellent in heat resistance, mechanical strength, electrical properties, optical properties, chemical stability etc., and used widely in industry by utilizing these properties. However, these materials are generally highly brittle and rigid, and their use may be limited because work or mechanical processing at a high temperature is necessary for working them into a desired shape.
On the other hand, organic polymers are excellent in workability and have flexibility, so that they can be easily worked into a desired shape. However, the organic polymers are often inferior to the inorganic materials in heat resistance and chemical stability.
Hence, attention is attracted in recent years to an organic-inorganic composite material comprising an inorganic material and an organic polymer material to attain the properties of the two.
As the composite material of an organic polymer and an inorganic material, a composite material having an inorganic material in a fibrous or powdery form dispersed in an organic polymer material has been used in various fields. In recent years, there is extensive development of an organic-inorganic nano-composite material (also called an organic-inorganic hybrid material) in which organic and inorganic regions are compounded at the nanometer level or molecular level.
Because the organic and inorganic regions can be dispersed at the nanometer level or molecular level, the organic-inorganic nano-composite material is used as a material for electronic components or as a material for mechanical components. Further, the organic or inorganic region in the material can be designed to be smaller than light wavelength, thus making light absorption and scattering low. Accordingly, the organic-inorganic nano-composite material has been studied to be used as a material for optical waveguide, optical fiber, or the like by providing optical transparency.
Methods of producing organic-inorganic composite materials are disclosed in “Structure of poly (vinylpyrrolidone)-silica hybrid”, Motoyuki Toki, et al., Polymer Bulletin 29, 653–660 (1992) and in “Organic-inorganic hybrid sol-gel materials, 1”, Jen Ming Yang, et al., Die Angewandte Makromolekulare Chemi 251 (1997) 49–60 (Nr. 4356) etc.
However, the production methods described in these literatures have a problem that organic-inorganic composite materials excellent in optical transparency cannot be obtained.
When the organic-inorganic nano-composite material is used as an optical waveguide, its light transmission layer is often formed on a substrate. In this case, the adhesion of the substrate to the light transmission layer is important.
For the purpose of improving the adhesion, it has been attempted to provide a graded structure with the material by continuously changing its composition.
For example, in Japanese Patent Laid-Open No. 34413(2000), proposed is a silica/polycarbonate-based composite material wherein the concentration of silica is continuously changed by applying successively a plurality of coating solutions different in the composition of organic and inorganic components on a substrate.
Further, in Japanese Patent Laid-Open No. 336281(2000), proposed is a graded structure prepared by applying a coating solution consisting of a mixture of an organic polymer and a metal compound capable of forming a metal oxide by hydrolysis on an organic substrate, then heating and drying it, the graded structure having a higher content of the organic component at the side of the substrate and a higher content of the inorganic component in the vicinity of the surface.
However, the methods proposed in the publications described above have a problem that the adhesion of the light transmission layer to the substrate cannot be sufficiently improved.